How does a glass barometer work?

There are many different ways to create a barometer, but they all break down into two different styles as described here:

What a barometer is measuring is the pressure in the atmosphere. A meteorologist is looking for an exact measurement. But generally speaking (there are a dozen exceptions...), there is a big picture view of pressure as well: when the air pressure is high the weather tends to be fair. When it is low, it is likely to be cloudy and rainy. An old-style glass barometer was a very simple way to see whether the pressure was high or low.

This video shows you one style of glass baraometer - it looks like a little tea pot: